Recently, novel high-throughput screening (HTS) devices based on microfluidics technology have been developed which have the capability to rapidly assay compounds for their effects on various biological processes, including, for example, fluorogenic and mobility shift assays for multiple target classes including kinases, proteases and phosphatases, cell-based assay including calcium flux and membrane potential, and the like. See, e.g., U.S. Pat. Nos. 5,942,443 and 6,132,685, both assigned to the assignee of the instant invention, the entire contents of which are incorporated by reference herein. Such HTS systems perform high volume experimentation, on the order of tens of thousands of experiments per microfluidic chip, using nanoliters of reagents. Such systems typically perform experiments in serial, continuous flow fashion and employ a “chip-to-world” interface, or sample access system, called a “sipper” through which test compounds residing in stacked microwell plates are sipped into a capillary or capillaries attached to the chip and drawn into the microfluidic channels of the chip. There they are mixed with the target biomolecule and a series of processing steps is carried out to determine the effect of the compounds on the target. The benefits of systems for high throughput screening are several: decreased time for assay development; assay transfer and hit validation; integrated reagent handling leading to enhanced productivity; significant reductions in compound library use; up to a 100,000-fold reduction in target use for screening; and higher quality, more reproducible results leading to more reliable hit and lead generation.
In operation of such HTS systems, typically multiwell plates containing large numbers of different test compounds are stacked on top of one another in a plate stack and then fed one at a time, e.g., by robotic systems (e.g., x-y or x-y-z-type robotic translation systems), conveyor systems, or the like, to a test area of the system where the samples in the multiple wells of the plates can be accessed by one or more sippers extending from the microfluidic chip, such that the test compounds can be sampled into the chips, e.g., interspersed by appropriate spacer fluid regions. After loading the test compounds into the chips, the multiwell plates are then collected or stacked at an opposite end of the system for retrieval.
The inventor has recognized that currently available multiwell plates often permit too much air to flow to the wells from the external environment. In a larger multiwell plate such as a 96 or 384 well plate, for example, such air exchange generally causes no problems with respect to the samples in the interior wells. However, those wells around the periphery or the corners of the multiwell plate are disturbed by excess airflow from the external environment because those wells have an interface to the ambient environment where the plate handling equipment is located. Conversely, the interior wells of the plate “see” an environment that is essentially saturated with water vapor (until the perimeter wells completely evaporate). Thus, evaporation theoretically proceeds in a stepwise fashion. That is, the perimeter wells evaporate completely before the interior wells show appreciable evaporation. Excess air causes the evaporation of samples from wells, which is an issue for the performance of assays, particularly in HTS systems, where run times may be on the order of eight hours or more. Samples must then either be replenished more frequently to compensate for evaporation, or the peripheral or corner wells filled with water or buffer and not used for the assays, which reduces the high throughput capabilities of the system.
To combat the problem of evaporation from multiwell plates, many conventional multiwell plates are designed with lids for covering the plates. An example of such a multiwell plate with lid assembly is exemplified in U.S. Pat. No. 4,657,867. Other multiwell plates use some form of a layer or membrane which is disposed over the vertical wells to prevent the evaporation of liquid therefrom, wherein the layer of film is penetrable, for example, by a pipette tip or the like to access one of the vertical wells to add liquid to or withdraw liquid from the well, as is described in U.S. Pat. No. 5,789,251, for example. However, in usage of HTS systems as described above, where multiwell plates are stacked one on top of another and must be readily and easily accessible by robotic systems and the like, the use of a separate lid or membrane to reduce evaporation is cumbersome and difficult to incorporate into the system.
Accordingly, when multiwell plates are stored in a stacked configuration (or even stored unstacked), as they typically are in usage in HTS systems, there is a need for a multiwell plate(s) which is capable of reducing the air flowing to the wells, so as to minimize evaporation of sample from the wells, particularly those wells which are located on the perimeter of the multiwell plate, while still allowing the wells to be readily accessible by automated sipper devices and the like using robotic systems typically employed in HTS systems. The present invention is directed to such multiwell plates which solve the above-described problems.